Dielectrics serve essential functions in devices, including microelectronic devices. Dielectric materials and films are electrical insulators which provide mechanical or structural support for electronic, mechanical, or electromechanical devices. Dielectric films are used to electrically and mechanically isolate electrical or microelectromechanical components from other device components or the outside environment. In this respect, the films must be of high mechanical quality and have a low or tunable dielectric constant. In addition, the ability to directly photo-pattern the dielectric film without the use of a temporary photoresist for pattern transfer is attractive due to the decreased time and cost requirements for device manufacturing. Negative-tone dielectrics often have good lithographic properties, but typically require organic solvents to develop the latent image. In addition, negative-tone materials often use a bright field mask where most of the mask is transparent. This leads to higher defect rates than dark field masks because particles on the mask have a high probability of resulting in component defects. Existing positive-tone dielectrics, however, have lithographic limitations. These limitations include poor patternability and low photo-speed because each chemical reaction requires at least one photon. Thus, thick dielectric films require very high optical doses.
Presently, the cost of lithographic tools significantly impacts on the cost of the final component. Furthermore, the nature of the developers used to develop the latent image also impacts the effect of the lithographic process on health, safety and environmental issues in the manufacture of such components. Organic solvents used to develop negative-tone, photosensitive dielectrics are often flammable, and are potentially hazardous to the health of the employees and to the environment. The use of aqueous developers is thus desirable because they are less hazardous to workers and the environment.
Polymers can generally provide the needed electrical, mechanical, and chemical properties with the added benefit of easy and low-cost processing. Some polymer dielectrics are photo-definable, which reduces the number and severity of the steps required to etch vias through the dielectric films. Positive-tone materials are more suitable for interlayer dielectric applications than negative-tone materials since positive-tone materials use mostly opaque masks, making photolithography less sensitive to particulates and increasing yield. Additionally, the ability to develop the latent image in an aqueous solution (e.g. aqueous base) reduces the need for environmentally harmful organic solvents.
A desirable combination of attributes for thick film, permanent dielectrics is to have positive-tone imaging, aqueous development, and high optical sensitivity and contrast. However, common diazoquinone (DQ)-containing, positive-tone materials have low sensitivity and photospeed due to the low quantum efficiency of DQ (Mack (1988) Applied Optics 27, 4913-4919). DQ also has a high absorption coefficient making exposure of thick films difficult. Fortunately, the absorption coefficient of the DQ photoproduct, indene carboxylic acid (ICA), is less than DQ, providing a photo-bleaching effect. However, thick films still require doses on the order of 100 to 1000 mJ/cm2 (Maier (2001) Progress in Polymer Science 26, 3-65; Vleggaar et al. (1994) Journal of the American Chemical Society 116, 11754-11763; Mueller et al. (2012) Journal of Applied Polymer Science doi:10.1002/app.38055).
Chemically amplified (CA) mechanisms are a route to improving the photospeed compared to DQ-based systems. The most popular positive-tone CA systems are made possible by an acid catalyzed deprotection of a pendent functional group to cause a developer solubility switch, making the exposed regions soluble in aqueous base developer (Reichmanis et al. (1991) Chemistry of Materials 3, 394-407; Ito (2000) IBM Journal of Research and Development 44, 119-130). A functional group, such as an acid or alcohol, is called protected when it is in a different chemical form which can be readily converted into the deprotected form, acid or alcohol in this example. For example, tert-butoxycarbonyl (TBOC) and tert-butyl ester (TBE) are the protected forms of an alcohol and a carboxylic acid, respectively. When the TBOC or TBE is converted into an alcohol or carboxylic acid, respectively, it is called the action of “deprotection”. The addition and exposure of a small amount of a photoacid generator (PAG) can result in multiple deprotection reactions. The most common CA systems involve the deprotection of a TBOC or a TBE moiety to produce an alcohol or a carboxylic acid, respectively. The resulting deprotected groups are soluble in aqueous base developer whereas the unexposed regions remain insoluble. The high optical sensitivity of these mixtures enables thick film, positive-tone polymer films with good lithographic properties.
In order to obtain good mechanical and electrical properties in a permanent dielectric, it is often necessary to cross-link the polymer film. Cross-linking is the formation of a chemical bond, often a covalent bond between two previously unbounded moieties within the polymer mixture so that the average molecular weight of the polymer increases. Cross-linking usually improves the chemical or mechanical properties of the polymer, and can decrease its solubility in a solvent or developer. However, problems can arise with cross-linking in CA chemistries because many cross-linking mechanisms are acid catalyzed. For example, epoxy cross-linkers readily ring open in the presence of an acid and react with alcohols and carboxylic acids to form base-insoluble ethers and esters at low temperature, respectively (Raeis-zadeh, et al. (2011) Journal of Applied Polymer Science 120, 1916-1925; Parker & Isaacs (1959) Chemical Reviews 59, 737-799). Exposure and baking of a positive-tone CA film with multifunctional epoxy additives would cause the deprotection and immediate cross-linking of the exposed regions, leaving the exposed regions insoluble in the base developer.
Thus, there remains a need for chemical functionalities and a mechanism to enable preparation of positive-tone, aqueous-developable, CA, cross-linkable dielectrics.